The NLI is binding with the institution, not the coach _ If the coach leaves the institution (voluntarily or not) the provisions of the NLI remain in effect. The NLI does NOT guarantee a spot on the team _ The NLI only guarantees that athletic aid will be awarded for one academic year. All decisions on rosters and playing time are rendered by the coaching staff at the institution.
The difference is a Letter of Intent is basically a letter stating the kind of offer you are contemplating in making. Usually it contains what I call the GENERAL items of price, terms and closing date. Most of the other legal STUFF is left out.